Field of the Invention
The present invention relates to a light scanning apparatus that includes a light intensity control portion configured to control the intensity of a light beam.
Description of the Related Art
Some known light scanning apparatus provided in an image forming apparatus use a method of irradiating a photosensitive drum via an fθ lens with a light beam that is deflected by a rotary polygon mirror after exiting a light source. In recent years, image forming apparatus have been demanded to form a high quality image at high speed and, to meet the demand, use as a light source a multi-beam light source, which is configured to emit a plurality of light beams concurrently from a plurality of light emitting points.
Meanwhile, light scanning apparatus switch the rotation speed of the rotary polygon mirror and the number of light emitting points of the light source in response to a change in printing speed (variable speed), a change in image resolution, or a change in the rotation speed of the photosensitive drum which depends on the thickness of the recording medium in a manner that suits the new printing speed, the new image resolution, or the new drum rotation speed. In Japanese Patent No. 5629975, there is disclosed light intensity control under which light for an image forming area on a surface of the photosensitive drum is emitted from a smaller number of light emitting points that is suited to image data, and light for a non-image forming area on the drum surface is emitted from all light emitting points on light emission schedules different from one another.
However, in the case where the number of light beams is switched by using laser drive circuit boards of the same type in which control for supplying a bias current to a plurality of light emitting points is executed in order to improve the light beam output response for different types of light scanning apparatus, a bias current is supplied also to a light emitting point that is not in use. This presents a difficulty in reducing the power consumption of the light scanning apparatus. When the light source used is a VCSEL or another light source that emits a large number of light beams, power consumption due to a bias current supplied to light emitting points that are not in use is particularly large, which is a problem. This is one of cases where it is desired to reduce a bias current supplied to a light emitting point that is not in use. Meanwhile, there are cases where it is desired to increase a bias current as close to the threshold current (light emission start current) of a light emitting point as possible in order to prevent an image defect (a fog) resembling scumming, which appears due to an accidental development of a slight amount of toner in a white portion (unexposed portion) where no printing is supposed to take place. There are also cases where it is desired to increase the bias current as close to the threshold current of a light emitting point as possible in order to improve the light beam output response in high-speed image forming.